1. Field of the Invention
The present invention relates to an image forming apparatus, e.g. a copier or printer, implementing a method for counting the number of “clicks,” i.e. the number copies that have been made.
2. Description of Background Art
In image forming apparatuses such as printers, copiers and multipurpose reproduction devices, it is frequently required to count the number of hard copies that have been made, e.g. for the purpose of determining suitable maintenance intervals, or, more importantly, for billing purposes. In conventional apparatus, mechanical counters are used for that purpose because such counters are known to be very robust and reliable. In particular, mechanical counters will retain their count values even in case of an unexpected power breakdown of the machine. On the other hand, since mechanical counters are relatively expensive, it would be desirable to replace them with electronic counters, while preserving a comparable level of robustness and reliability.
U.S. Pat. No. 4,774,544 discloses an image forming apparatus, wherein a method of the type described above is used for counting the number of copies. The “clicks” are counted by a CPU of the machine control, which comprises a non-volatile memory. An EEPROM (Electrically Erasable Programmable Read Only Memory) is provided as a non-volatile memory for saving the count values to make them persistent during those times when the power supply for the CPU is turned down. The contents of an EEPROM can be read as often as desired, but can be erased and re-written only a limited number of times. Since this number is significantly smaller than the number of copies that is expected to be made and to be counted during the total lifetime of the image forming apparatus, this EEPROM is subdivided into a plurality of memory areas each of which can store a complete count value, so that the storage capacity of the EEPROM is multiplied. Each time when a “click” is counted, the new count value is written in one of the memory areas of the EEPROM, and when the number of erase and write cycles of that memory area becomes exhausted, the future count values will be written into another memory area.
U.S. Pat. Nos. 5,568,626 and 5,450,460 disclose other counting methods dealing with the problem of the limited number of write cycles of an EEPROM.
However, all these methods require a considerable amount of storage capacity of the non-volatile memory, which increases the costs for these memory devices. Moreover, these known methods address the problem that the count values that are stored in the non-volatile memory may become corrupted only to a limited extent. Therefore, for example, when a machine error leads to an unexpected shutdown or when a power failure of the machine control system takes place at the very moment when new data are written into the EEPROM, the write procedure will be disturbed and the value that will be written into the memory area of the EEPROM becomes unpredictable. A similar problem will occur when an EEPROM or a specific memory area thereof reaches the end of its lifetime (earlier than expected).
U.S. Pat. No. 4,665,497 describes an odometer wherein a traveled distance is counted in a volatile counter, and certain increments of that distance, e.g. 100 m, are saved in a non-volatile memory having a plurality of memory areas. Another count value is saved when the power is switched off. When power is switched on again, the counter is initialized with the maximum of the count values read from the memory areas.